1. Field of the Invention
The present invention is directed to a transmission method for an analog magnetic resonance signal from a reception coil to a reception circuit that is spatially separated from the reception coil. The present invention also is directed to a corresponding reception coil for the reception of an analog magnetic resonance signal and to a magnetic resonance system.
2. Description of the Prior Art
A transmission method of the above type, and a reception coil and magnetic resonance system corresponding to are known from German OS 41 26 537.
A frequency conversion (single side-band modulation) ensues in German OS 41 26 537. In such a frequency conversion, the signal-to-noise ratio of the signal converted back onto the Larmor frequency corresponds to the signal-to-noise ratio of the transmission path. The method according to German OS 41 26 537 therefore exhibits the disadvantage that considerable efforts must be undertaken in order to assure the signal-to-noise ratio required in magnetic resonance applications.
The object of the present invention is comprised in creating a transmission method and the devices corresponding therewith with which a high signal-to-noise ratio can be achieved with optimally little outlay.
This object is achieved in a transmission method wherein a carrier signal with a modulation signal corresponding to the magnetic resonance signal is frequency-modulated around a carrier frequency at the coil side, the frequency-modulated carrier signal is wirelessly transmitted from the reception coil to the reception circuit, and the frequency-modulated carrier signal is demodulated in the reception circuit in order to reacquire the modulation signal.
The above object is achieved in a reception coil having a frequency-variable oscillator with a control input and a transmission device, the control input being supplied with a modulation signal corresponding to the magnetic resonance signal, a carrier signal of the frequency-variable oscillator being frequency-modulated around a carrier frequency on the basis of the supplied modulation signal, and the frequency-modulated carrier signal being supplied to the transmission device, with which it can be wirelessly transmitted.
An inventive reception system of a magnetic resonance installation is composed of at least one such reception coil and at least one reception circuit for a carrier signal modulated in this way, the reception circuit has an antenna device for receiving the carrier signal, and the modulated carrier signal can be supplied to a demodulator with which the modulation signal can be reacquired.
When the magnetic resonance signal is converted onto an intermediate frequency in a frequency converter and the output signal of the frequency converter corresponds to the modulation signal, the frequency of the modulation signal can be reduced in a simple way. The demands of the dynamics to be made on the frequency-variable oscillator thus can be reduced.
When the intermediate frequency is considerably lower than the Larmor frequency of the magnetic resonance signal, a greater modulation index, and thus an especially high signal-to-noise ratio derives given the same frequency boost of the carrier signal. This is particularly true when the intermediate frequency lies between 1 and 3 MHz.
When the modulation signal is compressed before the frequency-modulation of the carrier signal, the signal-to-noise ratio is improved particularly given small signal amplitudes at which the noise is particularly disturbing.
When the reacquired modulation signal is digitized in the reception circuit and the digitized modulation signal is decompressed, the decompression function can be realized in an especially simple way.
When the frequency-modulated carrier signal transmitted wirelessly from the reception coil to the reception circuit is received via a number of antennas at the reception circuit side and is supplied to the reception circuit via a selection device, the antenna via which the signal is supplied to the reception circuit can be selected. A good reception of the frequency-modulated carrier signal thus is achieved when a signal fade occurs at the location of one of the antennas due to multi-path receptionxe2x80x94which one must count on. In this case, a switch simply can be made to a different antenna.
When the carrier frequency of the unmodulated carrier signal is variable, the same components can be utilized at different carrier frequencies. A number of reception systems thus can be simultaneously operated at the same magnetic resonance installation without mutually disturbing one another.